The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the production of temporal phase images in a fast cardiac MRI acquisition capable of being acquired in a single breath-hold.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins, and after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Most NMR scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts. There is a class of pulse sequences which have a very short repetition time (TR) and result in complete scans which can be conducted in seconds rather than minutes. When applied to cardiac imaging, for example, a complete scan from which a series of images showing the heart at different phases of its cycle can be acquired in a single breath-hold as described in co-pending U.S. patent application Ser. No. 102,166, filed on Aug. 4, 1993 and entitled "MRI Cardiac Image Produced By Temporal Data Sharing".
The number of cardiac phase images that can be acquired during a scan is determined by a number of factors such as pulse sequence repetition time, the number of views acquired at each cardiac trigger and the patient's heart rate. By decreasing the number of views acquired at each cardiac trigger, more phase images can be acquired and the "temporal resolution" of the series of cardiac images is increased. However, total scan time is also increased because the number of cardiac triggers required to complete the acquisition of each phase image is increased.
Coronary arteries are usually surrounded by pericardial fat. The high NMR signal produced by this fat can at times obscure the NMR vascular signal which is of most interest to the clinician. To improve the detectability of the coronary arteries, therefore, it is desirable to utilize a fat suppression sequence such as that disclosed in U.S. Pat. No. 5,256,967 issued on Oct. 26, 1993 and entitled "Fast NMR Image Acquisition With Spectrally Selective Inversion Pulses". Unfortunately, such fat suppression pulse sequences significantly increase the time required to acquire each set of views, resulting in fewer cardiac phase images or a longer scan time.